It is relatively simple to determine net O2 flux in leaves. However, this data provides no information on the underlying processes responsible for this flux, namely gross oxygen production (GOP, water splitting), mitochondrial respiration in light, Rubisco oxygenation, and photorespiration. In this ‘Breakthrough Technologies’ paper, Gauthier et al outline a method that utilises H218O labelling to precisely measure both net oxygen production (NOP) and GOP for detached leaves. This was performed in a chamber with regulated temperature and humidity, and conventional CO2 and light response curves were performed simultaneously. As a proof of concept, the authors used this technique to determine O2 metabolism in leaves of Phaseolusvulgaris, and examined both the light response of mesophyll CO2 conductance and the Kok effect. (Summary by Mike Page) Plant Physiol 10.1104/pp.16.00741